Look in the mouth. Painful superficial ulcers, aka canker sores.
Life. Up to 2/3 of people will get them in a lifetime. They tend to start in childhood and fade over time. They are non-contagious, non-infectious, and not an STD.
Who knows for sure, there are a lot of alleged reasons for idiopathic oral ulcers. Causes of the non-idiopathic variety of mouth ulcers include herpes, histoplasmosis. If it is painless, think syphilis.
AIDS patients, who can get large aphthous ulcers, topical and systemic steroids may be of help as can thalidomide 100-200 mg q.d. for up to six weeks.
In normal people, topical steroids.
Relevant links to my Medscape blog
Last Update: 05/07/18.